Add multi-target support

[pincher95]

PR: Add multi-target /probe endpoint + auth-modules (fixes #1062)

This PR implements the feature described in issue #1062 (“multi-target scraping”).

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What’s new
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1. /probe endpoint
   Accepts ?target= and (optionally) auth_module=.
   For each request the exporter spins up a throw-away Prometheus registry, registers the usual collectors with the supplied ES URL, gathers, and streams metrics.

2. Auth-module support (--config.file=)
   Simple YAML schema modelled after postgres_exporter & mysqld_exporter.

   auth_modules:
     prod_basic:
       type: userpass
       userpass:
         username: metrics
         password: s3cr3t
   
     staging_key:
       type: apikey
       apikey: BASE64IDKEY==
   
     # options: <k:v> are appended as DSN query parameters when present

3. Alias on /metrics
   Requests that already contain target= are internally forwarded to the probe handler, so users may keep path: /metrics in ServiceMonitors if they wish.

4. Per-module API key support
   type: apikey injects the Authorization: ApiKey … header for that probe only.

5. Docs & examples

   • README: updated multi-target section with full table of supported auth types.
   • examples/auth_modules.yml shows userpass, apikey.

6. Unit tests

   • config loader round-trip.
   • Validation edge cases for validateProbeParams.

───────────────────────────────
Compatibility
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• Single-target usage unchanged – /metrics without target= still returns the startup cluster.
• All existing CLI flags (TLS, indices, AWS signing, …) still apply.
• No exported metrics renamed.

───────────────────────────────
Prometheus sample config
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- job_name: es-multi
  metrics_path: /probe
  params:
    auth_module: [prod_basic]
  static_configs:
    - targets:
        - https://es-prod:9200
    - targets:
        - https://es-staging:9200
      labels:
        __param_auth_module: staging_key
  relabel_configs:
    - source_labels: [__address__]
      target_label: __param_target
    - source_labels: [__param_target]
      target_label: instance
    - target_label: __address__
      replacement: exporter:9114

───────────────────────────────
Commit outline
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1. refactor: probe handler, alias logic
2. feat: YAML auth-module loader (userpass, apikey)
3. docs: README + example file
4. tests: config loader & validation
5. CHANGELOG entry

Feedback / suggestions welcome!

[sysadmind]

I need to spend more time with the code to provide all of my feedback, but want to provide what I have ready while I gave it the first pass.

[sysadmind]

I think the guts of this are really solid. It did take me some time to give a proper review so once the changes are pushed up, I will need to allocate some time to re-review. Thanks for submitting this, I think it will be a good improvement.

[sysadmind]

All of the new files are also missing the copyright assignment/license header. You should be able to copy paste (make sure there are no dates) from any other go file. I don't love the extra requests for the two collectors but I think that can be cleaned up after #1052 is merged.

[sysadmind]

The DCO check is also failing on at least one commit

[sysadmind]

Thanks! Great stuff! I appreciate all of your hard work on this one to get it ready to ship.